In recent years, with the continuous development of the technical level of the printing and packaging industry, the product quality has been continuously improved. Especially in the fiercely competitive tobacco industry, in order to enhance the artistic visual effects of cigarette packs, various processes are constantly being updated, and its design level, printing quality, and post-press processing have also been continuously improved. Among the various post-printing processes of cigarette pack products, the bronzing process is popular with people because of its unique surface finishing effect and is widely used.
Principle and characteristics of bronzing
Hot stamping uses the principle of hot press transfer to transfer the aluminum layer in anodized aluminum to the surface of the substrate to form a special metal effect. Because the main material used for bronzing is anodized aluminum foil, so bronzing is also called anodized aluminum hot stamping. Anodized aluminum foil is usually composed of multiple layers of materials, the substrate is often PE, followed by separation coating, color coating, metal coating (aluminum plating) and glue coating.
The basic process of bronzing is under pressure, that is, in the state where the anodized aluminum is pressed by the hot plate and the substrate, the anodized aluminum is heated to melt the hot-melt silicone resin layer and the glue on it. The viscosity of the silicone resin becomes smaller, and the viscosity of the special heat-sensitive adhesive increases when it is heated and melted, so that the aluminum layer and the anodized aluminum base film are peeled and transferred to the substrate at the same time. As the pressure is removed, the adhesive quickly cools and solidifies, and the aluminum layer is firmly attached to the substrate to complete a hot stamping process.
Bronzing has two main functions: one is surface decoration, which can increase the added value of the product. The combination of bronzing and other processing methods such as embossing and embossing technology can show the strong decorative effect of the product: the second is to give the product a higher anti-counterfeiting performance, such as the use of holographic positioning hot stamping trademark logos.
After bronzing, the pattern is clear, beautiful, colorful and eye-catching, wear-resistant and weather-resistant. At present, the bronzing process on printed tobacco labels accounts for more than 85%. In graphic design, bronzing can play a finishing touch and highlight the design theme. It is especially suitable for the decoration of trademarks and registered names.
The development of bronzing technology
In recent years, with the improvement of the processing level of bronzing materials and bronzing plates and the use of new machinery and equipment, the domestic bronzing process has also received some new developments. High-speed bronzing, three-dimensional bronzing, holographic positioning bronzing and printing after bronzing are widely used in cigarette pack production.
Three-dimensional bronzing: three-dimensional bronzing technology is a combined technology of bronzing technology and concave-convex embossing technology. It uses corrosion or engraving technology to make the bronzing and embossing graphics into a negative and positive concave and convex mold. Achieve the process of hot stamping and bump embossing technology at one time. This process simultaneously completes bronzing and embossing, which reduces the waste products caused by the inaccurate processing procedures and overprinting, and improves production efficiency and product quality.
Because the product effect formed by three-dimensional bronzing is a relief-shaped three-dimensional pattern, it can no longer be printed on, so it must be designed after printing first and then hot. At the same time, due to its high precision and high quality requirements, it is not suitable for cold stamping. Technical production. The three-dimensional bronzing equipment usually has two types: flat flattening and round flattening. The round flattening method is not suitable for three-dimensional bronzing.
The flat pressing flat three-dimensional hot stamping equipment is represented by BOBST equipment, which is developing in the direction of high speed, high precision, and convenient use. It is the main model of three-dimensional hot stamping. Bost hot stamping machine has reached a hot stamping speed of 7,500 sheets / hour, and the accuracy can be kept within 0.1 ~ 4mm, which is one of the most advanced hot stamping machines.
The method of round pressing and round bronzing is usually used for web-line processing. Because the round pressing is a line contact, the bubble phenomenon that may exist in the flat pressing and bronzing can be completely avoided, and the speed is fast and the quality is good. However, because the bronzing version and the bottom die relief are round, the processing is difficult and the cost is high. The heat transfer effect of the circular bronzing version is not very ideal, so it is not widely used. It is often used in some large output and high quality long Edition work, such as cigarette packs, wine bags, health products, etc.
Due to the popular flexible packaging of high-end cigarettes, the original post-press processing technology used on white cardboard has also been transferred to soft paper. The process of soft paper embossing and positioning hot stamping completes hot stamping and embossing in one process. The product has the characteristics of full hot stamping, fine graphics, beautiful appearance, and no embossing and skew. However, this process is difficult, and the precision requirements for hot stamping plates, holographic films, and hot stamping machines are extremely high, and the various operating indicators are very strict. Because of this, it is difficult to be imitated and the degree of anti-counterfeiting is high.
Hologram bronzing: Laser holograms are made according to the principle of laser interference and using spatial frequency coding. Laser holograms are widely used in the field of anti-counterfeiting due to their good color, obvious layers, vivid images, good optical transformation effects, and high information technology content.
The latest optical variable pattern technology has been clearly distinguished from the widely used HOLOGRAM hologram (three-dimensional hologram), and some holograms already have machine recognition functions. The mechanism of holographic hot stamping is that the hot melt adhesive layer and the separation layer on the holographic hot stamping material are heated and melted by the hot stamping die on the hot stamping equipment, and under a certain pressure, the information layer of the hot stamping material is holographic grating The stripe is separated from the PET base material, so that the aluminum foil information layer is bonded to the ironing surface and melted into one body to achieve a perfect combination.
Holographic hot stamping mainly includes ordinary hot stamping and positioning hot stamping. Among them, the special version of hologram positioning hot stamping has frequently appeared on high-end cigarette packs in the past two years. It refers to that the hologram of a specific part of the holographic anti-counterfeiting hot stamping anodized aluminum is accurately stamped on a specific position of the cigarette pack by photoelectric identification on the hot stamping equipment. The hologram positioning technology is very difficult, not only requires printing plants to be equipped with high-performance and high-precision special positioning equipment, but also requires high-quality special positioning hot stamping anodized aluminum, and the production process must be strictly controlled to produce qualified Cigarette products.
Cold bronzing process: Cold bronzing technology is the first new technology achievement in the European and American printing industry in recent years. It is to complete the transfer of anodized aluminum foil under cold pressing. Cold bronzing uses special anodized aluminum, the back of which is not coated with adhesive. The adhesive is directly applied to the location where decoration is needed during printing. During the transfer, under the action of the adhesive, the anodized aluminum adheres to the surface of the printed product.
The cold bronzing process first prints UV ink. After drying, the special UV curing adhesive is printed on the ink surface where gold foil is needed; then the moisture in the adhesive is evaporated through the UV drying device and a thin layer of high-viscosity adhesive is formed. Then, under the action of a pair of metal rollers, the aluminum foil is brought into contact with the adhesive and pressed together. Due to the large adhesive force between the adhesive and the aluminum foil, when discharging, the part where the aluminum foil contacts the adhesive remains on the surface of the printed product and is integrated with the printed product, while the other parts of the aluminum foil are discharged like traditional bronzing. Thus completing the first part of the bronzing technology. Since the aluminum foil pattern transferred on the printed matter floats on the surface of the printed matter, the firmness is poor, so the surface of the printed matter must be polished or coated to protect the aluminum foil pattern.
Compared with the traditional hot stamping process, the cold stamping process is fast and saves energy: the material has a wide range of applications, and cold stamping can be performed on thermal paper and some film materials. The general flexographic printing plate is used to replace the expensive metal plate cylinder, so the production cost and cycle of the bronzing plate are greatly reduced, and the pollution during the corrosion of the metal plate is also reduced.
The cold bronzing process is limited by the conditions of adhesives and metal foils, and also has some disadvantages. For example, the printed products must be glazed or coated after cold bronzing to protect the bronzing patterns, which increases the cost and complexity of the process. The cold foiling process is not suitable for application on shrinkable PVC, PE and other film materials. Due to the poor leveling of the UV adhesive, the surface smoothness after drying is poor, which leads to the poor brightness of the metal foil surface transferred thereon, and the phenomenon of diffuse reflection, which is not as good as the traditional hot stamping process. The key technology of the cold bronzing process is UV adhesives and special anodized aluminum metal foil, which has broad development prospects.
First hot printing process: In order to highlight the specific patterns and logos on cigarette packs, in recent years, some printing houses have introduced a new process of hot stamping anodized aluminum and then printing patterns on the anodized aluminum, which can make the theme of the cigarette pack more Vivid and lively.
Printing first and then printing is a new idea in the industry of cigarette label design and printing. The application of this process has high requirements for anodized aluminum. The key of the process is that the hot stamping of anodized aluminum must be firm. During the printing, the anodized aluminum must not be pulled back by the printing ink. Anti-pulling not only affects the quality of the cigarette pack, but also the anodized aluminum that is pulled back will quickly accumulate on the blanket, ranging from the loss of the image dots to the partial printing of the image. As the machine runs, the anodized aluminum powder also It will be transferred to the ink roller system to accelerate the wear of the ink roller. Therefore, this process should select the appropriate anodized aluminum according to the characteristics of the printing paper to ensure that the anodized aluminum can be firmly attached to the surface of the paper.
In addition to the requirements of accurate bronzing position, smooth and bright surface, balanced and uniform pressure, no blistering, no paste version in production, it also requires that there should be no obvious indentation on the edge of the bronzing pattern. Good adhesion and no obvious scratches and scratches. Everyone knows that the bronzing process is actually a process in which the paper is heated and compressed. For white cards and glass cards, special attention should be paid to the protection of semi-finished products, and various adverse factors of paper deformation should be minimized during the production process, which will greatly help the smooth progress of the post-gilding process and the improvement of product qualification rate.
Micro-embossing process: This process is to use micro-embossing on the aluminum foil paper graphics and text to apply various fine lines. The obtained product has the effect of hot stamping and embossing, and the metal texture is very strong. [next]
Bronzing quality control
At present, despite the emergence of various new types of bronzing processes, the traditional anodized aluminum hot bronzing technology is an important process in post-press processing. With its unique decorative effects and mature process operations, it has not been left out, but has been more popular More attention and preference.
The traditional thermoelectric piezoelectric aluminum stamping foil mainly adopts the method of heating and pressing to transfer the pattern or text to the surface of the material to be stamped. To complete this process, you need a hot stamping machine, a template with special text or patterns, the temperature required for heating, the pressure required for transfer and the corresponding time. It should be noted that when stamping different materials, the appropriate model of anodized aluminum should be selected, and the appropriate temperature, pressure, and stamping time should be selected to achieve the desired stamping effect.
Bronzing: High-quality bronzing is the primary factor to ensure the quality of bronzing. At present, the production of bronzing plate mainly adopts photo-etching technology and electronic engraving technology, and the material is mostly copper plate or zinc plate. The copper plate material is exquisite, and the surface finish and heat transfer effect are better than the zinc plate. The use of high-quality copper plate can improve the gloss and outline clarity of bronzing graphics.
The traditional photo-etching technique for making bronzing plates is simple, low in cost, and has a short service life. It is mainly used for bronzing of text, thick lines, and general images: for bronzing plates that are finer, uneven in thickness, etc., a second rotting Or use electronic engraving technology. The bronzing version produced by electro-engraving can express rich and delicate level changes, which greatly expands the expressive capacity of the packaging. This process is beneficial to environmental protection, but the investment in electric engraving equipment is relatively large. At present, the depth of engraving is not ideal, which is easy to cause bronzing "pasting", but its high printing resistance is suitable for long-type live parts of cigarette packs. The production technology of holographic anti-counterfeiting bronzing plate is relatively high, and the plate-making period is relatively long. It is only used for the packaging of fixed and large-volume products.
Hot stamping equipment: The selection of hot stamping equipment is a key factor in determining the quality of hot stamping. The sheet-foil stamping machine has three types: flattening, round flattening, and round flattening. The flattening flattening machine is currently the most widely used.
The three stamping methods have their own advantages and disadvantages. The round pressing and round pressing hot stamping implements the linear pressure, and the total pressure is small. It can easily complete the large area solid stamping with a relatively small pressure and the operation is stable. The round pressing type has higher production efficiency and is suitable for hot stamping of large quantities of live parts. Due to the difficulty of processing the copper plate arc surface and the high production cost, the heating roller is also more difficult to heat than the flat surface. Flat and flat operation is flexible and convenient, and is more suitable for short version products.
Compared with imported hot stamping equipment, domestic hot stamping equipment has obvious advantages in performance and price, but the overprint accuracy, stability, and function of imported machines are significantly better than domestic machines, and they are also more durable than domestic machines. machining.
Bronzing suitability: The so-called bronzing suitability refers to the selection of anodized aluminum models and the matching between the bronzing speed, temperature, pressure and other factors. In the case of fixed hardware such as equipment, correctly grasping the suitability of bronzing is the most important means to improve the quality of bronzing. The process of hot stamping is: hot stamping preparation, one version, one pad version, adjusting hot stamping process parameters, one hot stamping, one sample and one formal hot stamping.
It can be seen from the process of hot stamping that the three basic elements of hot stamping are: temperature, pressure and hot stamping time. In order to obtain the ideal hot stamping effect, the process parameters such as hot stamping temperature, hot stamping pressure, and hot stamping speed must be reasonably mastered and matched. In addition, the quality of the raw materials related to hot stamping must also be guaranteed, such as: the suitability of the hot stamping of the substrate, the performance of the anodized aluminum material, and the quality of the hot stamping plate. If something goes wrong in one link, it will affect the final hot stamping effect.
Hot stamping failure and troubleshooting
Hot stamping is not possible or the stamping is not true: This is related to the surface characteristics of the printed parts, the nature of the anodized aluminum, the hot stamping temperature and the pressure and other factors.
â‘ Too much powder is sprayed on the surface of the printing, the printing ink is not dry or the surface contains additives such as tackifier and varnish, which will hinder the adsorption of the anodized aluminum and the paper. It can be used to remove powder on the surface of the printed product or be resolved in the printing process.
â‘¡ Improper selection of anodized aluminum will also directly affect the fastness of bronzing. The type of anodized aluminum used should be comprehensively considered according to the size of the gold foil area and the characteristics of the material to be stamped. There are many models of anodized aluminum materials on the market, and each model corresponds to a substrate with different materials and its most suitable hot stamping range. They are mainly distinguished by the adhesion and peeling degree during use. When the operator selects the anodized aluminum material with poor fastness and tightness for hot stamping, the hot stamping pattern on the cigarette pack will appear to be unrealistic and fall.
When anodized aluminum is selected, the following categories can be referred to: hot stamping on general products (general ink color). Anodized aluminum has 88-1 type I and KURZ PM type: dense ink-colored prints (including gold and silver) for cigarette packs and cosmetics The hot stamping anodized aluminum has 88-2 type; the cigarette stamping, cosmetic packaging and other fine stroke hot stamping anodized aluminum has 88-3, 88-4 type, PM288, etc. Suitable for 0PP or PET laminated paper and cardboard with UV ink , Glazing paper and other products hot stamping anodized aluminum have 88-4 type, K series, LK series, and SP series.
â‘¢ The matching between the hot stamping equipment, hot stamping time, and hot stamping temperature is not properly controlled, which affects the fastness of hot stamping and the clarity of the graphic outline.
Due to the different equipment and materials to be stamped, the stamping time and stamping temperature are different. For example, the high-speed circular rounding machine is fast and the stamping line contacts, and the hot stamping temperature is higher than the circular flattening or flattening. In general, the temperature of round pressing and round hot stamping is 190% ～ 220 ℃, the round pressing is about 130 ℃ ～ 150 ℃, and the flat pressing is about 100 ℃ ～ 120 ℃. Of course, the pressing time, hot stamping temperature and production efficiency are also largely restricted by the transfer performance of anodized aluminum. Also pay attention to check whether the pressure is balanced, and check whether the hot stamping version is damaged if some graphics and text are not hot stamped.
Anti-pull: Anti-pull means that the anodized aluminum will remove the printing ink or varnish on the printed parts after hot stamping. The main reason is that the ink on the surface of the printed product is not dry or the post-processing of the UV on the surface of the printed product is improper, resulting in the poor combination of the ink and UV oil on the surface of the printed product and the surface of the paper.
The stamping should be done after the printed matter is dry. In addition, anodized aluminum materials with low separation force and excellent heat transfer can be used.
The discoloration and paste of anodized aluminum after paste and hot stamping are mainly caused by the poor production of the bronzing plate, the loose installation of the anodized aluminum, or the incorrect movement of the anodized aluminum foil. The discoloration of anodized aluminum after hot stamping is mainly caused by too high hot stamping temperature. In addition, the wrinkling of anodized aluminum is also easy to cause uneven coloring and discoloration of hot stamping, which can be solved by appropriately lowering the temperature. For the round flattening model, a fan can be added at the foil feeding place to keep the foil pulling and flutter, to avoid the anodized aluminum touching the bronzing version and burning it before hot stamping.
The glossiness of the surface of the anodized aluminum is destroyed, and the outline of the picture and text is blurred: hot stamping that requires lamination processing, because of fear that the gold foil is easy to fall off, first hot stamping and then laminating. This will easily cause the film (especially the matt film) to destroy the surface gloss of the anodized aluminum, and it is not suitable to use a water-soluble glue coating, otherwise it will cause the black surface of the anodized aluminum, and it is easy to cause the gold powder to stick to the edges of the text phenomenon. After hot stamping, it is depressed due to the pressure, and the glue is not easy to penetrate the surface of the anodized aluminum, which is easy to cause the separation of 0PP and paper at the hot stamping site and affect the quality of the product. The correct process is to cover the film before bronzing, and choose the anodized aluminum that matches OPP.
In short, there are many factors that affect the quality of bronzing. Making a high-quality bronzing version and correctly mastering the suitability of bronzing are the keys to improving the quality of bronzing. The production of cigarette packets is different from other publications. It has unique printing and processing requirements. The requirements for quality, decoration, and anti-counterfeiting are relatively high, and the batch printing volume is large. Before production, enterprises should carefully read the construction order, do daily maintenance and maintenance of the machine, master the performance of the electrochemical aluminum foil, and be familiar with the relationship between temperature, pressure, and speed. Only in this way can the quality of the cigarette pack bronzing be guaranteed.
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